This invention relates generally to wafer slicing and, particularly, to improved quality control in wafer slicing by accurately measuring waviness of the sliced wafers.
Most processes for fabricating semiconductor electronic components start with monocrystalline, or single crystal, silicon in the form of wafers. In general, semiconductor wafers are produced by thinly slicing a single crystal silicon ingot. After slicing, each wafer undergoes a number of processing operations to shape the wafer, reduce its thickness, remove damage caused by the slicing operation, and to create a highly reflective surface.
Known wafering processes include slicing an ingot into individual wafers with a cutting apparatus, such as a wire saw. In general, the wire saw uses a wire mounted on rollers for cutting the ingot. The drive mechanism of the wire saw moves the wire back and forth in a lengthwise direction around the rollers at an average speed of, for example, 600-900 meters per minute. Commonly assigned U.S. Pat. No. 5,735,258, the entire disclosure of which is incorporated herein by reference, discloses a wire saw apparatus for slicing silicon wafers.
Unfortunately, the wafer slicing process typically produces undesirable surface characteristics on a wafer such as thickness variations, warp, saw marks and kerf loss. Total thickness variation refers to the difference between the maximum measured thickness and the minimum measured thickness of the wafer. Warp refers to the difference between a maximum variance and a minimum variance from a standard surface of the wafer. Kerf loss refers to the loss of thickness of the wafer generated during cutting the wafer. ADE Corporation of Westwood, Mass. sells a non-contacting electric-capacity type sensor for characterizing wafer geometry (e.g., measuring total thickness variation and warp) under the trademark UltraGage.RTM. 9500. ADE Corporation also sells a wafer flatness measurement system under the trademark Galaxy AFS-300.TM..
The wafer slicing process also produces a waviness characteristic on the surface of the wafers, especially in wafers cut with a wire saw. In general, waviness refers to relatively localized surface variations. Although waviness is analogous to local changes in warp, presently available systems for measuring warp are not well suited for quantifying waviness. The undesirable surface characteristics resulting from wafer slicing usually can be satisfactorily reduced or eliminated by presently available post-slicing processing operations. However, such operations result in material loss and time inefficiencies. Therefore, further improvements in wafering processes are still desired. For example, measurements for waviness are needed to better understand this surface characteristic and to improve processes for reducing and/or eliminating waviness. Such measurements are also desired for determining whether a wafer has an unacceptable amount of waviness before undergoing further wafering processes. By reducing the problems associated with waviness, the wafering process may be simplified and its cost reduced.